Frequency stabilization of an oscillator usually requires a high Q circuit coupled to an oscillating device. In some known transistor oscillators, such as described in U.S. Pat. No. 2,825,813, issued Mar. 4, 1958 to J. C. Sperling and U.S. Pat. No. 3,611,206, issued Oct. 5, 1971, to Y. Miyake et al, a piezoelectric crystal is coupled between the emitter and collector electrode circuits of the oscillating transistor. These piezoelectric crystals provide higher Qs and higher frequency stability than is available from microwave cavity resonators. The frequency of operation of such known transistor crystal oscillators is of the order of a few MHz. However, such known arrangements are inadequate for directly stabilizing microwave oscillators in the range of 2 to 5 GHz. Furthermore, these known transistor crystal oscillators require complex and expensive circuitry including several capacitances and inductances.
A known high frequency transistor oscillator is described in U.S. Pat. No. 2,926,312, issued Feb. 23, 1960 to F. A. Brand et al., wherein a parallel-resonant tank circuit is coupled to the collector electrode of the transistor, and a variable capacitance is coupled between the parallel-resonant tank circuit and the emitter electrode of the transistor. The variable capacitance provides means for varying the amount of feedback between the tank circuit and the emitter-base circuit of the oscillator. Although this known transistor oscillator may generate harmonic output power at frequencies as high as 3.4 GHz, the complexity, size and difficulty in accurately tuning and ajusting the various variable circuit elements of the proposed arrangement render this transistor oscillator inadequate as a compact, low-cost, frequency stabilized microwave oscillator.
Use of dielectric resonators in microwave filters is described in U.S. Pat. No. 3,840,828, issued Oct. 8, 1974, to D. F. Linn et al. and assigned to the present assignee. In this known filter arrangement, a dielectric resonator is disposed over a stripline conductor such that the magnetic field lines of the stripline pass through the planar parallel surfaces of the resonator to a maximum degree. This known arrangement operates satisfactorily for its intended purpose, i.e., as a microwave filter, wherein neither microwave active elements nor feedback paths are required to perform the filtering function.